This invention relates to a method of making metal 8-quinolinolato complexes.
Aluminum tris(8-quinolinolates), particularly tris(8-hydroxyquinolinato) aluminum (AlQ), have become standard emitter and electron transport materials for small molecule organic light emitting diodes (OLEDs) such as those described in U.S. Pat. No. 5,061,569 and C. H. Chen, J. Shi, and C. W. Tang, xe2x80x9cRecent Developments in Molecular Organic Electroluminescent Materials,xe2x80x9d Macromolecular Symposia, 1997, 125, 1-48. Other metal 8-quinolinolato complexes have also been shown to be useful electron transport materials for OLEDs. Reported methods for making these compounds have required moisture sensitive reagents such as metal alkoxides and metal halides, which may require special handling procedures. In addition, these methods may not provide products of sufficient purity for the fabrication of OLEDs without the need for special purification techniques such as train sublimation (Wagner et al., J. Mat. Sci., 17, 2781 (1982)). Such purifications are difficult to perform on a large scale and pose a significant barrier to the large scale manufacturing of OLEDs.
Because of the high potential demand for AlQ, a simple method for large-scale, high purity synthesis of this compound, and related metal 8-quinolinolato complexes, without the use of air- and moisture-sensitive reagents, would be desirable. Disclosed herein are novel methods for making metal 8-quinolinolato complexes using easily-handled, inexpensive, and commercially available metal carboxylates such as aluminum lactate, aluminum stearate, and zinc acetate.
One aspect of the invention features the preparation of metal 8-quinolinolato complexes by the reaction of metal carboxylates with 8-hydroxyquinoline compounds represented by Formula I: 
where R6=hydrogen and R1-R5=hydrogen, halogen, cyano, alkyl, aryl, alkoxy, aryloxy, part of a fused aromatic or aliphatic ring system, and each R1-R5 group may incorporate further substituents.
Another aspect of the invention features bis and tris metal (8-quinolinolate) complexes.
Another aspect of the invention features a method of making aluminum tris(8-quinolinolates) comprising combining an aluminum (III) carboxylate material with at least three equivalents of an 8-hydroxyquinoline compound represented by Formula I in an appropriate organic solvent. The carboxylate may be, for example, aluminum stearate or aluminum lactate and the organic solvent may be, for example, ethanol or toluene.
Another aspect of the invention features the production of mixed-ligand metal 8-quinolinolato complexes from the reaction of metal carboxylates with a mixture of different 8-hydroxyquinoline compounds represented by Formula I, including mixed-ligand aluminum 8-quinolinolato complexes from the reaction of aluminum carboxylates and 8-hydroxyquinoline compounds represented by Formula I.
Another aspect of the invention features reaction compositions comprising metal carboxylate material and an 8-hydroxyquinoline represented by Formula I in an organic solvent.
An advantage of at least one embodiment of the present invention is that due to the air and moisture stability of the starting materials used for the synthesis of metal (8-quinolinolates) no inert atmosphere or specially dried solvents are required.
An advantage of at least one embodiment of the present invention is that the reagents are inexpensive and commercially available.
An advantage of at least one embodiment of the present invention is that the reactions described herein can be carried out in essentially any apparatus compatible with hot organic solvents.
An advantage of at least one embodiment of the present invention is that the synthesis reaction is rapid.
An advantage of at least one embodiment of the present invention is that if long reaction times can be tolerated, it may not be necessary to heat the reactions so long as the reactants have sufficient solubility in the chosen solvent.
An advantage of at least one embodiment of the present invention is that high yields of a clean product is obtained in the absence of difficult and lengthy separation and purification processes such as sublimation. The purity of the product allows its immediate incorporation in articles such as organic light emitting diodes.
An advantage of at least one embodiment of the present invention is that the reaction method of the present invention is applicable to the preparation of many metal 8-quinolinolato derivative including those containing a mixture of 8-quinolinolato ligands.
Other features and advantages of the invention will be apparent from the following detailed description, and claims.
The present invention describes how metal quinolinato complexes may be made from metal carboxylates and certain 8-hydroxyquinoline compounds having the structure of Formula I: 
where R6=hydrogen and R1-R5=hydrogen, halogen, cyano, alkyl, aryl, alkoxy, aryloxy, part of a fused aromatic or aliphatic ring system, and each R1-R5 group may incorporate further substituents. It was found that, for the structure of Formula I, an R6 group larger than hydrogen sterically hindered the bonding of the 8-hydroxyquinoline. Examples of ligands represented by Formula I include 5-(diphenylamino)-8-quinolinol, 7,8,9,10-tetrahydro-6-methyl-4-phenanthridinol, 3,5,7-trichloro-8-quinolinol, 3-decyl-8-quinolinol, 5-[(nonyloxy)methyl]-8-quinolinol, 3-allyl-8-hydroxyquinoline, 4-phenanthridinol, benzo[f]quinolin-5-ol, 5-chloro-8-hydroxyquinoline, 5,7-dichloro-8-hydroxyquinoline, 5-methyl-8-hydroxyquinoline, and 4-methyl-8-hydroxyquinoline. 5-hydroxyquinoxaline and its derivatives are also useful in this invention as are 2-(2-hydroxyphenyl)benzoxazole, 2-(2-mercaptophenyl)benzoxazole, 2-(2-hydroxyphenyl)benzothiazole, 2-(2-mercaptophenyl)benzothiazole and 8-quinolinethiol and their derivatives.
The metal 8-quinolinolates made by the present invention may be represented by Formula II:
M(L)xxe2x80x83xe2x80x83II
where M is the metal, L is the ligand of Formula I, and x is 2 or 3.
Mixed ligand metal 8-quinolinolato complexes may also be made per the present invention. These complexes may be prepared by reacting a metal carboxylate with a mixture of 8-hydroxyquinoline compounds having the structure of Formula I in an appropriate relative ratio. For example, aluminum lactate may be added to a solution containing two equivalents of 8-hydroxyquinoline and one equivalent of 5-chloro-hydroxyquine to produce bis(8-hydroxyquinolinolato) aluminum.
Mixed ligand metal 8-quinolinolato complexes are important because they are generally more amorphous than their single ligand counterparts. Thus, vapor deposited small molecule OLEDs will be more stable and have a longer lifetime since the amorphous glass electron transport layers formed by the mixed ligand metal quinolinato complex will not crystallize as readily as their single ligand counterparts during OLED storage or operation. The mixed ligand quinolinato complexes will also have greater solubility in organic solvents making them more useful than their single ligand counterparts in solution-cast molecularly doped polymer OLEDs such as those described in J. Kido, xe2x80x9cOrganic Electroluminescent Devices Based on Polymeric Materials,xe2x80x9d Trends in Polymer Science, 1994, 2, 350-355.
An example of a reaction of the present invention is the preparation of AlQ by the reaction of aluminum (III) carboxylates, such as commercially available and inexpensive aluminum stearate ([CH3(CH2)16CO2]3Al, C54H105AlO6) and aluminum lactate ([CH3CH(OH)CO2]3Al, C9H15AlO9), with at least three equivalents of 8-hydroxyquinoline (C9H7NO) in a compatible organic solvent to give aluminum tris(8-quinolinolato) (C27H18AlN3O3) (AlQ), in nearly quantitative yields This type of reaction is represented by equation 1: 
where R is a branched, straight chain, or cyclic alkyl group having from 1 to 20, preferably 2 to 17 carbon atoms, wherein the alkyl or cycloalkyl groups can have one or more substituents such as hydroxy, ether, and halogen.
An example of a reaction represented by equation 1 is the preparation of AlQ by the reaction of aluminum lactate with 8-hydroxyquinoline in ethanol. In this preparation, both the aluminum lactate and 8-hydroxyquinoline are at least partially soluble in ethanol, while the resulting AlQ is not. A similar reaction of aluminum stearate with 8-hydroxyquinolinc in ethanol also gave high yields of AlQ, but the product was less pure.
Toluene proved to be a more suitable solvent for the reaction of aluminum stearate with 8-hydroxyquinoline because both starting materials and stearic acid are readily soluble in toluene while the AlQ product is only sparingly soluble, allowing for easy separation of the AlQ. The reaction was rapid, with fine, needle-like crystals of AlQ appearing in the reaction vessel as the reaction mixture was warmed. High yields were obtained.
Reaction of aluminum stearate with 8-hydroxyquinoline in refluxing heptane precipitated AlQ during the course of the reaction. However, the inventors have found that this precipitate also contained significant amounts of stearic acid byproduct.
One of skill in the art will recognize that some solvents are preferred for a given reaction and that using other solvents may provide poor results. For example, it was found that using toluene as a solvent for aluminum lactate resulted in a low yield due to the low solubility of aluminum lactate in toluene.
Aluminum lactate may generally be a preferred reactant over aluminum stearate. Both aluminum lactate and the lactic acid byproduct are very soluble in water, which simplifies their removal from any crude AlQ products. The lower molecular weight of aluminum lactate also allows the use of a smaller mass, compared to aluminum stearate, of starting material,which can afford a significant advantage in scaling-up and commercial production of AlQ.
The AlQ obtained as a precipitate from the reaction of 8-hydroxyquinoline with aluminum stearate in toluene appears, after collection by filtration, solvent washing, and drying, to be suitable for use in the fabrication of organic electroluminescent (OEL) films, such as those described in U.S. Pat. No. 4,720,432. If desired, AlQ can be further purified by common methods such as recrystallization, sublimation, or Soxhlet extraction.
A lamp using recrystallized AlQ prepared from aluminum lactate in ethanol was fabricated on a glass substrate using vapor deposition techniques. At 20 mA/cm2 the lamp operated at 7.8 V with a luminance of 686 cd/m2. This corresponded to an external quantum efficiency of 1.06%. These values were essentially the same as values obtained from a lamp made using commercially available sublimed AlQ. Thus, the method of the invention for the synthesis of aluminum tris(8-quinololates) can provide large quantities of AlQ suitable for the production of OEL films.
In addition to the aforementioned aluminum stearate and lactate, a variety of aluminum tris(carboxylates) may be useful for the present invention including aluminum octoate, aluminum palmitate, and aluminum oxalate. Basic aluminum carboxylates of the general structure (RCO2)2AlOH, such as boric acid stabilized aluminum acetate, may also be useful in this invention, where R is the same as described in equation 1.
Other metal 8-quinolinolato complexes can be prepared by the method of this invention. Examples of metal carboxylates that may be useful as starting materials include barium acetate, cadmium acetate, dyspropium acetate, indium acetate, neodinium(2-ethylhexanoate), samarium naphthanate, and zinc acetate.